About Graphite

About Graphite:

Graphite is a modern industrial mineral now widely regarded as a major global growth commodity for the 21st century. 
 
Graphite is one of two naturally occurring allotropes of crystalline carbon – the other being diamond. In contrast to diamond, graphite is soft, greasy to the touch and soils fingers and paper.

Types of Graphite                                                                                                                                                    
 
Graphite generally occurs in one of three forms: 
  • Microcrystalline or amorphous; 
  • Crystalline lump or vein; or 
  • Crystalline flake
Microcrystalline or amorphous type graphite is made up of aggregates of fine graphite crystals, which give the material a soft, black, earthy appearance. This material is usually hosted by quartzites, phyllites, metagreywackes and conglomerates. Amorphous graphite is defined as being finer than 40µm in diameter, but some trade statistics define the upper limit at 70µm. Generally, the 40 - 70µm is the limit of resolution of the human eye. Deposits with grades of over 80% carbon are considered to be economically viable.
 
Crystalline lump or vein type graphite is found as interlocking aggregates of coarse and/or microcrystalline platy, or less commonly, acicular graphite. The veins are hosted by igneous and metamorphic rocks, such as gneiss, schist, quartzite and marble.
 
Flake type graphite occurs as flat, plate-like crystals, with angular, rounded or irregular edges, with the crystals disseminated throughout originally carbonaceous metasediments. Flake graphite ranges in flake size from 1mm to 25mm, with an average size of 2.5mm. For commercial purposes, flake graphite is divided into coarse flake (150-850µm in diameter) and fine flake (45-150µm in diameter). Fine flake may be further subdivided into powder (-75µm), fine flake (75-100 µm) and medium flake (100-150µm). Impurities include minerals that are commonly found in metasediments – usually quartz, feldspar, mica, amphibole, garnet and calcite, with occasional amphiboles, pyrrhotite, pyrite and magnetite.




 
Geological settings favourable to graphite mineralisation:
 
Flake graphite occurs in host rocks such as quartz-mica schist, feldspathic or micaceous quartzite and gneiss. Flake graphite may also occur in metamorphosed carbonate rocks, though these occurrences are currently of little economic significance. Flake graphite deposits are usually strata bound, with individual beds or lenses ranging from 30cm to more than 30m thick, and extending for lengths of two kilometres or more. Ore bodies are normally tabular, occasionally lenticular, and occur locally as irregular bodies in the hinge zones of folds. Most economic deposits of flake graphite are of Archean to late Proterozoic age. These rocks may contain up to 90% graphite, although 10-15% graphite is a more typical grade for an ore body.


The Importance of Flake Size:

Determination of flake size will indicate the size at which the graphite may become liberated from the host rock. This is important as it will minimise the amount of crushing and grinding required prior to mineral separation. Flakes in the size range 250µm-1mm will command the highest prices, with fine graphite flakes (down to 125µm) also in some demand. An excess of graphitic fines will reduce the flake size and therefore the value of the final product. Also fine graphite will coat other minerals, which may then act as graphite during froth flotation and be recovered with the graphite concentrate. This thereby reduces the grade of the product. Mica will often occur interlayered with graphite, and may be difficult to remove during preparation. Fine material (such as clay and lateritic soil) may also coat the graphite. These minerals will complicate graphite processing.
 
 
Expandable Flake Graphite:

Expandable flake graphite is a form of graphite created by a process in which a second material is inserted between the graphene layers of a graphite crystal or particle, essentially expanding the material. The resulting graphite product has an overall decreased bulk density and an approximately 10-fold increase in surface area. Its new properties make ‘expanded’ graphite an important material for high temperature and high pressure applications. 
 
Importantly, not all graphite expands equally, if it expands at all. ‘Expansion’ capacity is specific to certain forms of natural graphite and differences in treatment type, particle/flake size, etc., can also all play a role in the overall expansion of the graphitic material. 

Some applications that utilise expandable flake graphite require high expansion ratio, whereas some applications require a low expansion ratio. An important material parameter that affects the expansion ratio is the particle/flake size of the expandable flake graphite. In general, particle size is directly proportional to expansion ratio. Large flakes typically have higher expansion ratios than smaller flakes. 
 
 
Uses of graphite:
 
The global demand for commercial graphite is growing and is expected to double within the next eight years. This growth profile is being driven by the increasing number of applications for graphite in technology and industry. The material has applications in electronics, nuclear reactors, manufacturing, aircraft and automotive production and in developing energy markets. Importantly, graphite is an essential component of the modern lithium ion battery, making it a key material in smart phones, tablets, laptops and electric cars. 
 
Graphite is also used to produce graphene – "the world's next wonder material.” Graphene is an allotrope of carbon, essentially a one-atom thick layer of graphite. Its weight and shape makes graphene desirable for uses in computer chips, laptops, optics and lasers etc.
 
Graphite, if it possesses the special property of ‘expandability,’ can also be further processed to produce ‘expanded’ graphite. ‘Expanded’ graphite is used to produce flexible graphite sheets and foils for manufacturing gaskets, packaging and other sealing materials in critical applications. In particular, it is useful in high temperature and high pressure environments and is also considered valuable in battery market. ‘Expanded’ graphite is highly valuable and highly sought after. 
 
Graphite is a highly valuable commodity and its unique physical and chemical properties make it difficult to substitute.
 
 
Graphite economics: Characteristics and processing
 
Mineralogical characterisation of graphite-bearing rocks should primarily aim to determine the graphitic carbon content and graphite flake size, as these two properties largely determine the economic value of the graphite. 
 
Mineralogical characterisation also provides the basis for the planning of laboratory beneficiation trials for determining the size and shape of the graphite flakes, their relationship to other minerals present and the likely liberation size. Laboratory processing trials of samples containing flake graphite should aim to establish the optimal separation method that maximises product recovery and grade. 

Recovery is the proportion of total graphite present in the head sample (start material) which reports to the concentrate. It is calculated using grade and yield data gathered during processing trials.
Grade is the graphitic carbon content and comprises head grade (grade of the starting material) and the concentrate grade (grade of the processed concentrate).
Yield is the proportion of the total weight of the starting material contained in the concentrate.
 
An assessment of the economic potential of samples from a deposit will require an indication of the likely processing scheme, including grade and recovery data over a range of flake sizes. The results of various use-related tests may also be required.
 
Graphite is relatively easy to separate from its host rock, although processing is required to produce material of high grades (+90%) and some graphite types require multiple processing steps to eliminate impurities.
 
 
Processing
The concentration of graphite from graphite-bearing material is dependent on the graphite flake morphology and liberation size. As graphite is resistant to weathering, it may often be pre-concentrated in graphite-bearing soil, but with graphite-bearing rock, the graphite needs to be liberated. Roller mills and cone grinders (or high-speed impact mills in larger-scale operations) produce a differential grinding effect that tends to concentrate the graphite in the coarser size fractions. Granular materials, such as quartz and feldspar, are ground into the fines, whereas graphite flakes slip through relatively unscathed. This however may be countered by graphite being ground by contact with the granular material during grinding to produce an even distribution of graphite. Care should be taken not to over grind the graphite as it will tend to smear out over the other minerals present.
 
Beneficiation
Examination of the size fractions by binocular microscope will indicate if the correct liberation size was chosen, as essentially all the graphite flakes will be free of the host rock. If a large proportion of graphite is 'locked' into the host rock, then attempts at separation will produce poor results and the larger size fractions may need to be ground further. If this is required, then the next largest sieve diameter is used as the new liberation size. For example, if the original liberation size chosen was 2mm, then 1mm is selected as the new liberation size (and so on down the sieve series 2mm, 1mm, 500µm, 250µm and 125µm). The process of grinding and microscopic examination is repeated until the correct liberation size is achieved. Head grade becomes economically important if flake size is small or liberation size is small.
 
 Flow Sheet